In order to artificially control the dispersion relation of electromagnetic waves, there is known a structure in which a material, a conductor patch or the like having different dielectric constants is periodically arrayed. In this kind of structure, a structure that possesses a dispersion relation having a bandgap in which propagation of electromagnetic waves is prohibited in a specified frequency band is called an electromagnetic bandgap (hereinbelow, EBG) structure.
When such an EBG structure is installed on a printed circuit board or a device package substrate, it will inhibit the propagation of surface waves that are produced on the substrate surface. For this reason, the EBG structure is employed in order to reduce the electromagnetic interference between the antennas or devices mounted on the printed circuit board.
This EBG structure functions as a magnetic wall that reflects incident electromagnetic waves in phase in the vicinity of the band gap frequency band. For this reason, by installing the EBG structure on the back surface of an antenna, it is possible to achieve a lower profile of the antenna while maintaining its radiation efficiency.
Generally, an EBG structure has a structure in which square conductor patches are periodically arranged in a two-dimensional manner on the surface of a dielectric layer such as a printed circuit board, and each patch and a conductor plane on the rear face of the dielectric layer are electrically connected by through-hole vias or the like. In an EBG structure, because the capacitance component formed between the patches and the inductance component formed from the vias function as an LC parallel resonant circuit, a band gap arises near the resonance frequency.
In order to make the band gap region of the EBG structure correspond to the low frequency band region, the inductance component should be increased. A common method of increasing the inductance component is to adopt a method that lengthens the through-hole vias. For this reason, as a result of making it correspond to the low frequency band, there has been the problem of the thickness of the EBG structure increasing.
Patent Document 1 given below discloses an EBG structure that solves this kind of problem. The EBG structure that is shown in FIG. 12 in Patent Document 1 has, between the patch layer and the conductor plane layer, an intermediate layer in which is disposed an inductance element such as a spiral inductor, and this patch, the inductance element, and the conductor plane are connected by vias. With such a structure, the band gap region is made to correspond to the low frequency band by increasing the inductance component without enlarging the EBG structure.